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JOSHUA TREE NATIONAL PARK, Calif. (AP) — Joshua trees, those spiky sentinels of the Mojave Desert, are having a blossom bonanza and nobody is quite sure why.

Millions of the trees have been bursting into bundles of greenish-white flowers in California, Arizona, Nevada and Utah. Just about every tree has bloomed this spring when usually far fewer do and they produce fewer flowers, biologists said.

"This is a once-in-a-lifetime phenomenon," Cameron Barrows, a research ecologist with the University of California, Riverside, told the Riverside Press-Enterprise.

Some biologists have suggested that the trees benefited from late-summer thunderstorms last year or cool winter weather, said David Lamfrom, California desert manager for the National Parks Conservation Association.

James Cornett, a desert biologist, blames two years of drought that he believes have stressed the trees.

"Stress can be an inducement for reproduction," Cornett said. "If it appears that they may not survive, one of the best strategies is to go out with an explosion of reproduction."

Besides the drought, Joshua trees are under stress because of the slowly warming planet, Cornett said.

He said flower and seed production is up more than 40 percent compared with the previous best season in the 25 years that he has been studying the plants.

The bloom is expected to last for several weeks, although some trees already are putting out seed pods.